Cryptosporidium parvum is a coccidian parasite which causes persistent, life-threatening diarrhea in patients with AIDS. Control of the disease is hampered by the lack of specific drugs effective against C. parvum. A considerable amount of data indicate that antibodies reactive with surface- exposed antigens on the infective sporozoite and merozoite stages of C. parvum can neutralize infectivity and provide significant protection against cryptosporidiosis. Based on these data, thi component project previously isolated the conserved, neutralization-sensitive sporozoite surface antigens (CPS-500, GP23 and GP25-200 in native form and produced expanded panels of neutralizing monoclonal antibodies (mAbs) against each. In the current funding period, the focus has been on defining the epitope specificity of these mAbs and the molecular mechanisms by which they neutralize sporozoite infectivity. These lines of investigation have been pursued to facilitate rational selection of multiple-epitope-specific mAb combinations in a strategy to enhance the efficacy of passive immunotherapy for intestinal cryptosporidiosis, and to characterize molecular targets for therapeutic intervention. In this 2 year renewal application, we propose to complete experiments necessary to test the following hypotheses: 1) intestinal C. parvum infection can be controlled by a neutralizing mAb formulation targeting multiple, distinct epitopes on the infective sporozoite and merozoite stages, and 2) neutralizing mAbs control infection by preventing sporozoite invasion of host cells, or by arresting their subsequent intracellular development, and can be used to define molecules involved in the pathogenesis of infection. To test these related hypotheses, the following specific aims will be completed; Specific Aim 1: Determine the most efficacious formulation of neutralizing mAbs targeting distinct epitopes of C. parvum sporozoites and merozoite for control of intestinal cryptosporidiosis. Specific Aim 2: Define molecular mechanisms of mAb-mediated parasite neutralization.